1. Field
The embodiment of the present invention relates to a memory system.
2. Description of the Related Art
As memories capable of storing mass data for use, variable resistance memories including cell arrays easily formable in three dimensions, such as a ReRAM (Resistance RAM) and an ion memory, have received attention. These memories operate using the positive/negative of voltages in bipolar operation basically. They may also operate using one directional voltage in unipolar operation, though, as in a phase change memory and a carbon nanotube (CNT). A memory using nanotubes (NT) such as the CNT utilizes fine variations in molecules and accordingly causes lower power consumption than the use of phase changes caused when phase change elements pass though states such as material melting. Therefore, it becomes one of powerful candidates capable of configuring mass file memories.
A cell array can be formed in three dimensions by stacking unit cell arrays each including alignments of plural memory cells of the cross point type (hereinafter also referred to as “CP cells”). In the case of the cell array thus structured, however, the increase in capacity accordingly requires massive circuits for driving selection lines, which worsen the cell share as a problem.
In addition, parallel simultaneous accesses to plural CP cells commonly connected to one selection line put an overload on that selection line. This is a common problem in memory systems including cell arrays of CP cells, making no exception even if CNTs are used.